The subject matter disclosed herein relates generally to magnetic resonance imaging (MRI) systems, and more particularly to ramping MRI systems.
In conventional MRI systems having superconducting magnets, the windings of superconducting wire forming the superconducting magnets are cryogenically cooled using a helium vessel to maintain the magnets below a critical temperature. For example, the windings of the superconducting magnet are immersed in a bath or vessel of liquid helium to maintain a temperature below the critical temperature for superconducting operation.
When the MRI system is energized, and in particular when the superconducting magnet is energized, commonly referred to as ramping, the Lorentz force on the conductors increases, causing small movements of the wire that can lead to localized frictional heating. The generated heat can overheat a localized area of the coil and create a normal zone, where the conductors (or wires) of the windings lose superconducting property and transfer to a normal resistive state. The normal zone will spread through the winding due to the Joule heat and the thermal conduction, which results in a quench event. The quench is accompanied by the rapid boil-off of helium escaping from the cryogen bath in which the magnet windings are immersed.
Each quench, followed by the re-fill and re-ramp of the magnet, is an expensive and time consuming event. The conductors of the superconducting magnet are specified to provide enough stability margin to avoid normal zone propagation from the point of localized heating. The stability margin for the windings during ramping can be increased, for example, by increasing the critical current of the wires forming the windings, adding high conductivity stabilizer material or improving the cooling of the wires. However, these methods add cost and complexity to the MRI system.